Cats with chemotherapy-induced complete remission of thymic lymphoma will be randomly assigned to no further therapy or treatment with antiserum against feline oncornavirus associated cell membrane antigen (FOCMA). Animals entered in the study will be monitored clinically and immunologically with various in vitro and in vivo assays including FOCMA antibody titers and blast transformation with CON A, lipopolysaccharide, FOCMA antigen, and FOCMA-specific antibody-dependent cell-mediated cytotoxicity. Cats with ALL will be treated with immunoglobulins prepared from anti-FOCMA or control sera and administered with and without fresh normal serum to try to determine whether other serum components influence the therapeutic activity of anti-FOCMA antibodies, if appropriate.